In a known arrangement of a steam boiler, water is fed into the boiler at a controlled rate and is heated in the boiler to convert the water to steam. The heat required to convert the water to steam is provided by a burner whose hot products of combustion are passed through ducts in the boiler and then exhausted. The steam boiler is controlled by a boiler control system, which receives information from sensors indicating, inter alia, the level of water in the boiler and the pressure of steam in the boiler, and which controls the flow rate of water into the boiler as well as sending a control signal to a burner control system that controls the burner. The burner control system controls, inter alia, the flow of fuel and gas to the burner head in dependence upon a demand signal received from the boiler.
The water that is fed in to the boiler will, even when pre-treated, generally contain some solids in solution. The steam boiler increases the concentration of the solution through the evaporation of steam. If the level of total dissolved solids is increased, precipitate begins to form on surfaces within the boiler, leading to premature boiler failure, slower heat exchange rates and a reduction in the efficiency of the steam boiler.
One factor that may be taken into account to ensure the steam boiler functions efficiently is the level of total dissolved solids in the water, which should be maintained below a predetermined and preselected maximum level. The level of dissolved solids may be reduced by blowing down the steam boiler. During blow down the concentrated boiler water is partly replaced by feed water. Blowing down the boiler frequently can keep the average total dissolved solids well below the predetermined maximum, but heat is lost with the water discharged from the steam boiler and the efficiency of the steam boiler is reduced. More efficient operation can be obtained by blowing down the steam boiler to maintain the average level of total dissolved solids at a value almost equal to the preselected maximum value.